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IDR                                                          P. Lapukhov
Internet-Draft                                           Microsoft Corp.
Intended status: Informational                                 A. Premji
Expires: January 8, 2013                                 Arista Networks
                                                            July 7, 2012


           Using BGP for routing in large-scale data centers
                 draft-lapukhov-bgp-routing-large-dc-00

Abstract

   Some service providers build and operate data centers at the size
   exceeding 100,000 servers.  In this document, those data-centers are
   referred to as "large-scale" to differentiate them from more common
   smaller infrastructures.  The data centers of that scale have unique
   set of network design requirement, with primary focus on operational
   simplicity and stability.

   This document attempts to summarize the authors' experiences in
   designing and supporting large data centers, using BGP as the only
   control-plane protocol.  The intent is to describe a proven and
   stable routing design that could be leveraged by others in the
   industry.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
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   This Internet-Draft will expire on January 8, 2013.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal



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   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   described in the Simplified BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Traditional data center designs  . . . . . . . . . . . . . . .  3
     2.1.  Layer 2 Designs  . . . . . . . . . . . . . . . . . . . . .  3
     2.2.  Fully routed network designs . . . . . . . . . . . . . . .  4
   3.  Document structure . . . . . . . . . . . . . . . . . . . . . .  5
   4.  Network design requirements  . . . . . . . . . . . . . . . . .  5
     4.1.  Traffic patterns . . . . . . . . . . . . . . . . . . . . .  5
     4.2.  CAPEX minimization . . . . . . . . . . . . . . . . . . . .  6
     4.3.  OPEX minimization  . . . . . . . . . . . . . . . . . . . .  6
     4.4.  Traffic Engineering  . . . . . . . . . . . . . . . . . . .  6
   5.  Requirement List . . . . . . . . . . . . . . . . . . . . . . .  7
   6.  Network topology . . . . . . . . . . . . . . . . . . . . . . .  7
     6.1.  Clos topology overview . . . . . . . . . . . . . . . . . .  7
     6.2.  Clos topology properties . . . . . . . . . . . . . . . . .  8
     6.3.  Scaling Clos topology  . . . . . . . . . . . . . . . . . .  9
   7.  Routing design . . . . . . . . . . . . . . . . . . . . . . . . 10
     7.1.  Choosing the routing protocol  . . . . . . . . . . . . . . 10
     7.2.  BGP configuration for Clos topology  . . . . . . . . . . . 10
       7.2.1.  BGP Autonomous System numbering layout . . . . . . . . 11
       7.2.2.  Non-unique private BGP ASN's . . . . . . . . . . . . . 12
       7.2.3.  Prefix advertisement . . . . . . . . . . . . . . . . . 13
       7.2.4.  External connectivity  . . . . . . . . . . . . . . . . 13
     7.3.  ECMP Considerations  . . . . . . . . . . . . . . . . . . . 14
       7.3.1.  Basic ECMP . . . . . . . . . . . . . . . . . . . . . . 14
       7.3.2.  BGP ECMP over multiple ASN . . . . . . . . . . . . . . 15
     7.4.  BGP convergence properties . . . . . . . . . . . . . . . . 16
       7.4.1.  Convergence timing . . . . . . . . . . . . . . . . . . 16
       7.4.2.  Failure impact scope . . . . . . . . . . . . . . . . . 16
       7.4.3.  Third-party route injection  . . . . . . . . . . . . . 17
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 17
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 17
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
   11. Informative References . . . . . . . . . . . . . . . . . . . . 18
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18





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1.  Introduction

   This document presents a practical routing design to be used in
   large-scale data centers, sometimes called hyperscale or warehousr-
   scale.  The most distinctive characterstic of these data center is
   having 100,000 or more end hosts connected to the network.  While
   historically only a few companies have been operating networks of
   that scale, recent trend in building large cloud data centers re-
   ignated interest in network designs to support deployment of this
   scale.  In contrary to more traditional data center designs, the
   approach proposed in this document does not depend on large Layer 2
   domains and instead uses routing at every level of the network.  The
   reason to make that choice is based on the unique set of design
   requirements, with primary focus on cost reduction.  Furthermore,
   analyzing the requirements the conclusion is that BGP best suits to
   accomplish this goal due primarily to its simplicity and broad vendor
   support.


2.  Traditional data center designs

   This section provides an overview of two types of traditional data
   center designs - Layer-2 and fully routed Layer-3 topologies.

2.1.  Layer 2 Designs

   In the networking industry, common design choice for data centers is
   using a mix of Ethernet-based Layer 2 technologies.  Network topology
   typically looks like a tree with redundant uplinks and three levels
   of hierarchy (see Figure 1) commonly named Core, Aggregation and
   Access.  To accommodate bandwidth demands, every next level has
   higher port density and bandwidth capacity.  In this document, the
   topology layers will be referenced as "tiers", e.g.  Tier 1, Tier 2
   and Tier 3 instead of Core, Aggregation or Access layers.

















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                       +------+  +------+
                       |      |  |      |
                       |      |==|      |           Tier1
                       |      |  |      |
                       +------+  +------+
                         |  |      |  |
               +---------+  |      |  +----------+
               | +-------+--+------+--+-------+  |
               | |       |  |      |  |       |  |
             +----+     +----+    +----+     +----+
             |    |     |    |    |    |     |    |
             |    |=====|    |    |    |=====|    | Tier2
             |    |     |    |    |    |     |    |
             +----+     +----+    +----+     +----+
                |         |          |         |
                |         |          |         |
                | +-----+ |          | +-----+ |
                +-|     |-+          +-|     |-+    Tier3
                  +-----+              +-----+
                   | | |                | | |
                 [Servers]            [Servers]


               Figure 1: Typical Data Center network layout

   IP routing is normally used only at the upper layers of the topology,
   e.g.  Tier 1 or Tier 2.  The main reasons for introducing such large
   (sometimes called stretched) Level 2 domains, are the following:

   o  Supporting legacy applications that may require direct Layer 2
      adjacency or use non-IP protocols
   o  Seamless mobility for virtual machines, to allow preserving IP
      address when a virtual machine changes physical host
   o  Simplified IP addressing - less IP subnets is required for the
      data-center
   o  Application load-balancing may require direct Layer 2 adjacency to
      perform some functions such as Level 2 Direct Server Return (DSR)

2.2.  Fully routed network designs

   Network designs that leverage IP routing down to the access layer
   (Tier 3) of the network gained some popularity, mostly due to
   improved network stability, scalability (by means of information
   hiding) and convergence times.  A common choice of routing protocol
   for data center designs would be an IGP, such as OSPF or ISIS.  As
   data centers grow in scale, and server count exceeds tens of
   thousands, those fully routed designs become more attractive.




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   BGP is the de-facto standard protocol for routing on the Internet,
   having wide support from network equipment vendors and being well-
   understood by network engineers world-wide.  However, it is not
   common to see BGP being used in data centers that employ fully routed
   network design.  There multiple reasons for that:

   o  BGP is perceived as "WAN protocol only" and often not being
      considered for enterprise or data center application
   o  BGP is believed to converge "slower" than traditional IGPs
   o  BGP is assumed to have a dependency on the presence of an IGP,
      which assists with recursive next-hop resolution
   o  BGP require a lot of configuration efforts as it does not support
      any form of neighbor auto-discovery

   In this document we argue benefits of choosing BGP as the single
   routing protocol, including acceptable convergence time.


3.  Document structure

   The remaining of this document is organized as following.  First the
   design requirements for large scale data centers are presented.
   Next, the document gives an overview of Clos network topology and its
   properties.  After that, the arguments for selecting BGP as the
   single routing protocols are presented.  Finally, the document goes
   over design detail and specific BGP policy features.


4.  Network design requirements

   This section describes and summarizes network design requirement for
   a large-scale data center.

4.1.  Traffic patterns

   The primary requirement when building an interconnection network for
   large number of servers is accommodating application bandwidth and
   latency requirements.  For long period of time, it was common to see
   traffic flowing mainly to and from the data center.  There were no
   intense (highly meshed flows) traffic patterns between the machines
   within the same tier.  As a result, traditional "tree" topology was
   sufficient to accommodate data flow, even with high oversubscription
   ratios in network equipment.  If more bandwidth was required, it was
   added by "scaling up" the network elements, e.g. by adding more line-
   cards or replacing existing devices with higher capacity switches.

   In contrast, large-scale data centers often host applications that
   generate large amount of server to server traffic, also known as



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   "east-west" traffic.  Examples of such applications could be compute
   clusters such as Hadoop or live virtual machine migration in "cloud"
   data-centers.  Scaling up traditional tree topology to match those
   bandwidth demands becomes either too expensive or impossible due to
   physical limitation.

4.2.  CAPEX minimization

   Cost of networking component alone (CAPEX) constitutes about 10-15%
   of total data center cost [GREENBERG2009].  Still, absolute numbers
   are significant, and hence the need to constantly drive cost of
   networking elements down.  This is normally accomplished in two ways:

   o  Unifying network elements, preferably using the same hardware type
      or even the same device.  This allows for bulk purchases with
      discounted pricing.
   o  Driving costs down by introducing diversity of networking vendors
      that may supply equipment for data center network

   In order to allow for vendor diversity, it is important to minimize
   the feature requirements for network equipment software.  In
   addition, the above strategy means that network equipment vendor
   choice may change often, or that the network may have to be multi-
   vendor and interoperability becomes critical.

4.3.  OPEX minimization

   Operating large scale infrastructure could be expensive, provide that
   larger amount of elements will statistically fail more often.
   Therefore, it is important to operate on the simplest software and
   feature set possible.

   An important aspect of OPEX minimization is reducing size of failure
   domains in the network.  Ethernet data-plane is known to be
   susceptible to massive impact due to broadcast or unicast storms.
   The use of fully routed designs reduces the size of data-plane
   failure domains, but at the time introduces the problem of
   distributed control-plane failures.  This requirement calls for
   simpler control-plane protocols that are expected to have less
   chances of network meltdown.

4.4.  Traffic Engineering

   In any data center, application load-balancing is critical function
   performed by network devices.  Traditionally, load-balancers are
   deployed as dedicated devices in traffic forwarding path.  A common
   problem is scaling load-balancers under growing traffic demand.
   Preferable solution would be able to scale load-balancing layer



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   horizontally, by adding more of the uniform nodes and distributing
   incoming traffic across them.

   In situation like this, an ideal choice would be using network
   infrastructure to distribute traffic across a group of load-
   balancers.  A combination of features such Anycast prefix
   advertisement [RFC4786] along with Equal Cost Multipath (ECMP)
   functionality could be used to accomplish this.  To allow for more
   granular load-distribution, it is beneficial for the network to
   support the ability to perform controlled per-hop traffic
   engineering.


5.  Requirement List

   This section summarizes the requirements in a list, based on the
   analysis made before

   o  REQ1: Select a network topology where capacity could be scaled
      "horizontally" by adding more links and network switches of the
      same type, without requiring upgrading the network elements
      themselves.
   o  REQ2: Define a narrow set of software features/protocols supported
      by multitude of networking equipment vendors.
   o  REQ3: Among the network protocols, select those having simpler
      implementation in terms of minimal programming code complexity.
   o  REQ4: The selected network routing protocol should support per-hop
      change of forwarding behavior.


6.  Network topology

   This section outlines the most common choice for horizontally
   scalable topology in large scale data centers.

6.1.  Clos topology overview

   A common choice for horizontally scalable topology is folded Clos
   topology (sometimes called "fat-tree").  This topology features odd
   number of stages (dimensions) and commonly made of the same uniform
   elements, e.g. switches of the same port count.  Therefore, the
   choice of Clos topology satisfies both REQ1 and REQ2.  See Figure 2
   below for an example of folded 3-stage Clos topology:








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             +-------+
             |       |----------------------------+
             |       |------------------+         |
             |       |--------+         |         |
             +-------+        |         |         |
             +-------+        |         |         |
             |       |--------+---------+-------+ |
             |       |--------+-------+ |       | |
             |       |------+ |       | |       | |
             +-------+      | |       | |       | |
             +-------+      | |       | |       | |
             |       |------+-+-------+-+-----+ | |
             |       |------+-+-----+ | |     | | |
             |       |----+ | |     | | |     | | |
             +-------+    | | |     | | |   ---------> M links
               Tier1      | | |     | | |     | | |
                        +-------+ +-------+ +-------+
                        |       | |       | |       |
                        |       | |       | |       | Tier2
                        |       | |       | |       |
                        +-------+ +-------+ +-------+
                          | | |     | | |     | | |
                          | | |     | | |   ---------> N Links
                          | | |     | | |     | | |
                          O O O     O O O     O O O   Servers

                  Figure 2: 3-Stage Folded Clos topology

   In the networking industry, a topology like this is sometimes
   referred to as "Leaf and Spine", where Spine is the name for the
   middle stage of the Clos topology (Tier 1) and Leaf is the name of
   input/output stage (Tier 2).  However, for consistency, the document
   will be using "Tier n" notation.

6.2.  Clos topology properties

   The following are some key properties of the Clos topology:

   o  Topology is fully non-blocking (or more accurately - non-
      interfering) if M >= N and oversubscribed by a factor of N/M
      otherwise.  Here M and N is the uplink and downlink port count
      respectively, for Tier 2 switch, as shown on Figure 2
   o  Implementing Clos topology requires a routing protocol supporting
      ECMP with the fan-out of M or more
   o  Every Tier 1 device has exactly one path to every end host
      (server) in this topology





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   o  Traffic flowing from server to server is naturally load-balanced
      over all available paths using simple ECMP behavior

6.3.  Scaling Clos topology

   Clos topology could be scaled either by increasing network switch
   radix or adding more stages, e.g. moving to a 5-stage Clos, as
   illustrated on Figure 3 below:

                                  Tier1
                                 +-----+
                                 |     |
                              +--|     |--+
                              |  +-----+  |
                      Tier2   |           |   Tier2
                     +-----+  |  +-----+  |  +-----+
       +-------------| DEV |--+--|     |--+--|     |-------------+
       |       +-----|  C  |--+  |     |  +--|     |-----+       |
       |       |     +-----+     +-----+     +-----+     |       |
       |       |                                         |       |
       |       |     +-----+     +-----+     +-----+     |       |
       | +-----+-----| DEV |--+  |     |  +--|     |-----+-----+ |
       | |     | +---|  D  |--+--|     |--+--|     |---+ |     | |
       | |     | |   +-----+  |  +-----+  |  +-----+   | |     | |
       | |     | |            |           |            | |     | |
     +-----+ +-----+          |  +-----+  |          +-----+ +-----+
     | DEV | | DEV |          +--|     |--+          |     | |     |
     |  A  | |  B  | Tier3       |     |       Tier3 |     | |     |
     +-----+ +-----+             +-----+             +-----+ +-----+
       | |     | |                                     | |     | |
       O O     O O            <- Servers ->            O O     O O


                      Figure 3: 5-Stage Clos topology

   The topology on Figure 3 is built from switches with radix 4 and
   provides full bisection bandwidth to all connected servers.  We'll be
   referring to the collection of directly connected Tier 2 and Tier 3
   switches as "cluster" in this document.  For example, devices A, B,
   C, and D on Figure 3 form a cluster.

   In practice, Tier 3 level of the network (typically top of rack
   switches, or ToRs) often introduces oversubscription to allow for
   packaging more servers in data center.  The main reason to
   oversubscribe only at a single layer of the network is to simplify
   application development that would need to account for two bandwidth
   pools: within the same access switch (e.g. rack) and outside of the
   local switch.  Oversubscription, however, does not affect routing



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   design and hence not considered in more details in this document.


7.  Routing design

   This section discusses the motivation for choosing BGP as the routing
   protocol and BGP configuration for routing in Clos topology.

7.1.  Choosing the routing protocol

   The set of requirement provide above calls for a single routing
   protocol (REQ2) in the data center to reduce complexity and
   interdependencies.  While it's common to rely on an IGP in this
   situation, the document proposes to use BGP only.  The advantages of
   using BGP are argued below.

   o  BGP has less complexity in protocol design - internal data
      structures and state-machines are simpler when compared to a link-
      state IGP.  For example, as opposed to implementing adjacency
      formation and maintenance, flow-control, etc.  BGP simply relies
      on TCP as the underlying transport.  This also simplified protocol
      testing and fulfills REQ1 and REQ2.
   o  BGP information flooding overhead is less when compared to link-
      state IGPs.  Indeed, since every BGP router normally re-calculates
      and propagates best-paths only, a network failure is masked as
      soon as BGP speaker finds an alternate path.  On contrary, event
      propagation scope of a link-state IGP is single area/domain,
      regardless of the failure type.  Furthermore, all well-known link-
      state IGPs feature periodic refresh updates, while BGP does not
      expire routing state.
   o  BGP supports third-party (recursively resolved) next-hops, which
      allows for injecting custom routing paths into any device in the
      network, using eBGP multi-hop peering session.  This satisfied
      REQ4 stated above.  Some IGPs, such as OSPF, support similar
      functionality using special concepts such as "Forwarding Address",
      but do not satisfy other requirement, such as protocol simplicity.
   o  BGP is easier to troubleshoot, mostly because of simplified
      protocol mechanics and database structures that directly map to
      forwarding tables structure.  For example, it is straightforward
      to dump contents of LocRIB and compare it to the router's RIB and
      FIB.  As another example, BGP routing updates translate directly
      into NLRI information, as compared to LSA/LSP information that
      describes network topology.  Thus BGP fully satisfies REQ3.

7.2.  BGP configuration for Clos topology

   This section provides configuration guidelines for a 5-stage Clos
   topology.  It is easy to reduce it to a 3-Stage Clos configuration,



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   and having topology that has more than 5 stages is very uncommon due
   to high link density of associated designs.

7.2.1.  BGP Autonomous System numbering layout

   The diagram below illustrates suggests BGP Autonomous System Number
   (BGP ASN) allocation scheme.  The following is a list of guiding
   principles:

   o  All BGP peering sessions are external BGP (eBGP) established over
      direct point-to-point links interconnecting the network switches.
   o  16-bit (two octet) BGP ASNs are used, for the reason of wider
      vendor support and better vendor interoperability (e.g. no need to
      support BGP capability negotiation).
   o  Private BGP ASNs from the range 64512-64534 are used for the
      reasons of avoiding ASN conflicts and being able to use BGP
      private ASN stripping feature (see below).
   o  A single BGP ASN is allocated to the Clos middle stage ("Tier 1"),
      e.g.  ASN 64534 on Figure 4
   o  Unique BGP ASN is allocated per every group of "Tier 2" switches.
      All Tier 2 switches in the same group share the BGP ASN.
   o  Unique BGP ASN is allocated to every Tier 3 switch (e.g.  ToR) in
      this topology.




























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                                ASN 64534
                               +---------+
                               | +-----+ |
                               | |     | |
                             +-|-|     |-|-+
                             | | +-----+ | |
                  ASN 64XXX  | |         | |  ASN 64XXX
                 +---------+ | |         | | +---------+
                 | +-----+ | | | +-----+ | | | +-----+ |
     +-----------|-|     |-|-+-|-|     |-|-+-|-|     |-|-----------+
     |       +---|-|     |-|-+ | |     | | +-|-|     |-|---+       |
     |       |   | +-----+ |   | +-----+ |   | +-----+ |   |       |
     |       |   |         |   |         |   |         |   |       |
     |       |   |         |   |         |   |         |   |       |
     |       |   | +-----+ |   | +-----+ |   | +-----+ |   |       |
     | +-----+---|-|     |-|-+ | |     | | +-|-|     |-|---+-----+ |
     | |     | +-|-|     |-|-+-|-|     |-|-+-|-|     |-|-+ |     | |
     | |     | | | +-----+ | | | +-----+ | | | +-----+ | | |     | |
     | |     | | +---------+ | |         | | +---------+ | |     | |
     | |     | |             | |         | |             | |     | |
   +-----+ +-----+           | | +-----+ | |           +-----+ +-----+
   | ASN | |     |           +-|-|     |-|-+           |     | |     |
   |65YYY| | ... |             | |     | |             | ... | | ... |
   +-----+ +-----+             | +-----+ |             +-----+ +-----+
     | |     | |               +---------+               | |     | |
     O O     O O              <- Servers ->              O O     O O


                 Figure 4: BGP ASN layout for 5-stage Clos

7.2.2.  Non-unique private BGP ASN's

   The use of private BGP ASNs limits to a range of 1022 unique numbers.
   It is possible that the number of network switches could exceed this
   value, and such situation requires a workaround.  One approach could
   be re-using the private ASN's assigned to Tier 3 switches across
   different clusters.  For example, private BGP ASN's 65001, 65003 ...
   65032 could be used within every individual cluster to be assigned to
   Tier 3 switches.

   To avoid Tier 3 route discards on the Tier 3 switches sharing the
   same ASN due to AS PATH loop prevention, upstream eBGP sessions on
   Tier 3 switches must be configured with so-called "AllowAS In"
   feature.  This BGP policy feature allows accepting device's own ASN
   in incoming BGP path advertisements.  Introduction of this feature
   does not create the opportunity for permanent routing loops under
   misconfiguration since AS PATH is always increments when routes are
   propagated from tier to tier.



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   Another solution to this problem would be switching over to using
   four-octet (32-bit) BGP ASNs.  However, there is no explicitly
   reserved private ASN range in four-octet numbering, but a work is in
   progress to request such an allocation in
   [I-D.mitchell-idr-as-private-reservation].  This will also require
   vendors to implement specific policy features, such as private AS
   removal from AS-PATH attribute.

7.2.3.  Prefix advertisement

   Clos topology has large number of point-to-point links and associated
   prefixes.  Advertising all of them into BGP may create FIB sizing
   issues, and there are two possible solutions to overcome this:

   o  Do not advertise any of the point-to-point links into BGP.  Since
      eBGP peering changes next-hop address at every node, this will not
      create any reachability issues for subnets advertised from Tier 3
      switches.
   o  Advertising point-to-point links, but summarizing them on every
      advertising device.  This requires proper address allocation, for
      example allocating a consecutive block of IP addresses per Tier 1
      and Tier 2 device to be used for point-to-point interface
      numbering.

   Server facing subnets on Tier 3 switches are announced into BGP
   without using summarization on Tier 2 and Tier 1 switches.
   Summarizing subnets in the Clos topology will result in route black-
   holing under a single link failure (e.g. between Tier 2 and Tier 3
   switch) and hence must be avoided.  The use of peer links within the
   same tier to resolve the black-holing problem is undesirable due to
   O(N^2) complexity of the peering mesh and waste of ports on the
   switches.

7.2.4.  External connectivity

   A dedicate cluster (or clusters) in Clos topology could be selected
   solely for the purpose of connecting to the Wide Area Network (WAN)
   edge devices, which we will call WAN Routers.  Tier 3 switches in
   such cluster would be replaced with WAN Routers, but eBGP peering
   will be used as usual, though WAN routers are likely to belong to a
   public ASN.

   The Tier 2 devices in such dedicated cluster will be referenced as
   "Border Routers" in this document.  These devices have to perform a
   few special functions:






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   o  Hide network topology information when advertising paths to WAN
      routers, i.e. remove some BGP AS-PATH information.  This is
      typically done to avoid BGP ASN number collisions across the data
      centers.  A BGP policy feature called "Remove Private AS" is
      commonly used to accomplish this.  This feature strips contiguous
      sequence of private ASNs found in AS PATH attribute prior to
      advertising the path to a neighbor.  This assumes that all BGP
      ASN's used for intra data center numbering are from private range.
   o  Originate a default route to the data center devices.  This is the
      only place where default route could be originated, as route
      summarization is highly undesirable for the "scale-out" topology.
      Alternatively, Border Routers may simply relay the default route
      learned from WAN routers.

7.3.  ECMP Considerations

   This section goes over Equal Cost Multipath (ECMP) functionality for
   Clos topology and covers a few special requirements.

7.3.1.  Basic ECMP

   ECMP is the key load-sharing mechanism leveraged by Clos topology.
   Effectively, every lower-tier switch will use all of its directly
   attached upper-tier devices to load-share traffic to the same prefix.
   Number of ECMP paths between two input/output switches in Clos
   topology equals to the number of the switches in the middle stage
   (Tier 1).  For example, Figure 5 illustrates the topology where Tier
   3 device A has four paths to reach servers X and Y, via Tier 2
   devices B and C and then Tier 1 devices 1, 2, 3, and 4 respectively.






















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                                  Tier 1
                                 +-----+
                                 | DEV |
                              +->|  1  |--+
                              |  +-----+  |
                      Tier 2  |           |   Tier 2
                     +-----+  |  +-----+  |  +-----+
       +------------>| DEV |--+->| DEV |--+--|     |-------------+
       |       +-----|  B  |--+  |  2  |  +--|     |-----+       |
       |       |     +-----+     +-----+     +-----+     |       |
       |       |                                         |       |
       |       |     +-----+     +-----+     +-----+     |       |
       | +-----+---->| DEV |--+  | DEV |  +--|     |-----+-----+ |
       | |     | +---|  C  |--+->|  3  |--+--|     |---+ |     | |
       | |     | |   +-----+  |  +-----+  |  +-----+   | |     | |
       | |     | |            |           |            | |     | |
     +-----+ +-----+          |  +-----+  |          +-----+ +-----+
     | DEV | |     | Tier 3   +->| DEV |--+   Tier 3 |     | |     |
     |  A  | |     |             |  4  |             |     | |     |
     +-----+ +-----+             +-----+             +-----+ +-----+
       | |     | |                                     | |     | |
       O O     O O            <- Servers ->            X Y     O O


               Figure 5: ECMP fan-out tree from A to X and Y

   The ECMP requirement implies that BGP implementation must support
   multi-path fan-out for up to the maximum number of devices directly
   attached at any point in the topology.  Normally, this number does
   not exceed half of the ports found on a switch in the topology, e.g.
   32 for a 64-port switch.

   Most implementations declare paths to be equal from ECMP perspective
   if they match up to and including step (e) in Section 9.1.2.2 of
   [RFC4271].  In the proposed network design there is no underlying
   IGP, so all IGP costs are automatically assumed to be zero (or
   otherwise the same value across all paths).  Loop prevention is
   assumed to be handled by BGP best-path selection process.

7.3.2.  BGP ECMP over multiple ASN

   For the purpose of application load-balancing purposes same prefix
   could be advertised from multiple Tier-3 switches.  From the
   perspective of other devices, such prefix would have BGP paths with
   different AS PATH attribute values, though having the same AS PATH
   length.  BGP implementation must support load-sharing for the paths
   having different AS PATH attribute values with equal attribute
   length.  This feature is sometimes known as "AS PATH multipath relax"



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   and effectively allows for ECMP to be done across different
   neighboring ASNs.

7.4.  BGP convergence properties

   This section reviews routing convergence properties of BGP in the
   proposed design.  A case is made that sub-second convergence is
   achievable provided that implementation supports fast BGP peering
   session shutdown upon failure of an associated link.

7.4.1.  Convergence timing

   BGP typically relies on IGP to route around link/node failures inside
   an AS, and implements either polling based or event-driven mechanism
   to obtain updates on IGP state changes.  The proposed routing design
   lacks any IGP, so the only mechanism that could be used for fault
   detection is BGP keep-alive packet exchange.

   Relying purely on BGP keep-alive packets may result in high
   convergence delays, on the order of multiple seconds (normally, the
   minimum recommended BGP hold time value is 3 seconds).  However, many
   BGP implementations can shut down local eBGP peering sessions in
   response to the "link down" event for the outgoing interface used for
   BGP peering.  This feature is sometimes called as "fast fall-over".
   Since majority of the links in modern data centers are point to point
   fiber connections, a physical failure translates into interface going
   down within order of milliseconds, and trigger BGP re-convergence.
   Furthermore, popular link technologies, such as 10Gbps Ethernet, may
   support simple form of OAM for failure signaling such as
   [FAULTSIG10GE], which makes failure detection more robust.
   Alternatively, as opposed to relying on physical layer for fault
   signaling, some platforms may support Bidirectional Forwarding
   Detection ([RFC5880]) to allow for sub-second failure detection and
   fault signaling to BGP process.  This, however, presents additional
   requirements to vendor software and possibly hardware, and may
   contradict REQ1.

7.4.2.  Failure impact scope

   BGP is inherently a distance-vector protocol, and as such some of
   failures could be masked if the local node can immediately find a
   backup path.  Worst case is that all devices would have to either
   withdraw a prefix completely, or update the ECMP paths in the FIB.
   That fault domain cannot be reduced by using summarization, since
   using this technique may create route black-holing issues as
   mentioned previously.





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7.4.3.  Third-party route injection

   BGP allows for a third-party BGP speaker (not necessarily directly
   attached to the network devices) to inject routes at any point of
   network topology.  This could be achieved by peering an external
   speaker using eBGP multi-hop session with some or even all devices in
   the topology.  Furthermore, BGP diverse path distribution
   [I-D.ietf-grow-diverse-bgp-path-dist] could be used to inject
   multiple next-hop for the same prefix and facilitate load-balancing.
   Using that technique, it is possible to implement unequal-cost load-
   balancing across multiple clusters in the data-center, by associating
   the same prefix with next-hops mapping to different clusters.

   For example, a third-party BGP speaker may peer with Tier 3 and Tier
   1 switches, injecting the same prefix, but using a special set of BGP
   next-hops for Tier 1 devices.  Those next-hops are assumed to resolve
   recursively via BGP, and could be, for example, IP addresses on Tier
   3 switches.  The resulting forwarding table programming could provide
   desired traffic proportion distribution among different clusters.


8.  Security Considerations

   The design does not introduce any special security concerns others
   than normally associated with BGP deployments.  For control plane
   security, BGP peering sessions could be authenticated using TCP MD5
   signature extension header [RFC2385].  Furthermore, BGP TTL security
   [I-D.gill-btsh] could be used to reduce the risk of session spoofing
   and TCP SYN flooding attacks against the control plane.


9.  IANA Considerations

   There are no considerations associated with IANA for this document.


10.  Acknowledgements

   This publication summarizes work of many people who participated in
   developing, testing and deploying the proposed design.  Their names,
   in alphabetical order, are George Chen, Parantap Lahiri, Dave Maltz,
   Edet Nkposong, Robert Toomey, and Lihua Yuan.  Authors would also
   like to thank Jon Mitchell for reviewing and providing valuable
   feedback on the document.







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11.  Informative References

   [RFC4786]  Abley, J. and K. Lindqvist, "Operation of Anycast
              Services", BCP 126, RFC 4786, December 2006.

   [RFC4271]  Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
              Protocol 4 (BGP-4)", RFC 4271, January 2006.

   [RFC2385]  Heffernan, A., "Protection of BGP Sessions via the TCP MD5
              Signature Option", RFC 2385, August 1998.

   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD)", RFC 5880, June 2010.

   [I-D.ietf-grow-diverse-bgp-path-dist]
              Raszuk, R., Fernando, R., Patel, K., McPherson, D., and K.
              Kumaki, "Distribution of diverse BGP paths.",
              draft-ietf-grow-diverse-bgp-path-dist-07 (work in
              progress), May 2012.

   [I-D.mitchell-idr-as-private-reservation]
              Mitchell, J., "Autonomous System (AS) Reservation for
              Private Use", draft-mitchell-idr-as-private-reservation-00
              (work in progress), June 2012.

   [I-D.gill-btsh]
              Gill, V., Heasley, J., and D. Meyer, "The BGP TTL Security
              Hack (BTSH)", draft-gill-btsh-02 (work in progress),
              May 2003.

   [GREENBERG2009]
              Greenberg, A., Hamilton, J., and D. Maltz, "The Cost of a
              Cloud: Research Problems in Data Center Networks",
              January 2009.

   [FAULTSIG10GE]
              Frazier, H. and S. Muller, "Remote Fault & Break Link
              Proposal for 10-Gigabit Ethernet", September 2000.













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Authors' Addresses

   Petr Lapukhov
   Microsoft Corp.
   One Microsfot Way
   Redmond, WA  98052
   US

   Phone: +1 425 7032723 X 32723
   Email: petrlapu@microsoft.com
   URI:   http://microsoft.com/


   Ariff Premji
   Arista Networks
   5470 Great America Parkway
   Santa Clara, CA  95054
   US

   Phone: +1 408-547-5699
   Email: ariff@aristanetworks.com
   URI:   http://aristanetworks.com/





























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